Multi-wavelength semiconductor lasers are needed for several applications, particularly for wavelength-division multiplexing (WDM) telecommunications systems including computer buses. In such systems, multiple lasers each emitting at a different frequency are separately modulated by different data signals, and all the so modulated optical carders are impressed on a single optical fiber. At the receiving end of the fiber, the separate optical wavelengths are separated by a spectrometer or other wavelength-sensitive means so that each single optical channel can be extracted from the fiber and detected. The absorption spectrum of the silica fiber used in long-distance telecommunication networks dictates that the optical carriers be in the 1.3 or 1.5 .mu.m bands available in InP and related active opto-electronic materials while the shorter computer buses can use the 0.8 .mu.m band available in GaAs.
For economy and ease of operation, the multiple lasers should be integrated on a single integrated circuit chip, that is, an opto-electronic integrated circuit (OEIC). Chang-Hasnain has disclosed a method of fabricating an array of vertical-cavity, surface-emitting lasers emitting at different wavelengths in U.S. Pat. No. 5,029,176. Although her method allows the fabrication of a large number of individual lasers, present designs for communication networks do not foresee a need for more than 20 to 40 separate wavelengths separated by about 1 or 2 nm. Her design suffers from doubts about its reproducibility, its incompatibility with wafer-scale fabrication, and the vertical emission of the laser light. It would be preferred to retain a planar geometry for ease of packaging.
A different approach uses distributed feedback (DFB) edge-emitting lasers in which separate Bragg diffraction gratings determine the lasing wavelengths. The period of the gratings are tailored for the separate wavelengths. The state of the art in this approach, presently about 20 lasers on a single integrated circuit chip, is disclosed by Zah et al. in "1.55 .mu.m tensile-strained single quantum well 20-wavelength distributed feedback laser arrays", Electronics Letters, vol. 28, 1992, pp.1585-1587. This approach suffers from two disadvantages. First, if the laser integrated circuit is to attain a channel spacing of about 1 nm, then the variation in the periods of the diffraction gratings and other portions of the structure must be controlled to about this same distance. The etching and lithography becomes very difficult in satisfying such tight dimensional control. Secondly, the efficient coupling of the multiple laser emissions into a single optical fiber remains unsolved. Of course, bulk optical lenses could be used to focus the outputs to the small fiber core, but such an approach would be neither economical nor rugged.
Several groups have proposed fabricating an OEIC spectrometer which could be integrated with multiple detectors for a WDM application. See, for example, Gibbon et al. in "Optical performance of integrated 1.5 .mu.m grating wavelength-demultiplexer on InP-based waveguide," Electronics Letters, vol. 25, 1989, pp. 1441-1442, Soole et al. in "Monolithic InP/InGaAsP/InP grating spectrometer for the 1.48-1.56 .mu.m wavelength range," Applied Physics Letters, vol. 58, 1991, pp. 1949-1951, and Cremer et al. in "Grating spectrograph in InGaAsP/InP for dense wavelength division multiplexing" Applied Physics Letters, vol. 59, 1991, pp. 627-629. In these approaches, multiple waveguides are formed in the surface of an OEIC. One of the waveguides acts as an input waveguide receiving light from off the chip. The input light exits the input waveguide on its interior end and irradiates a vertically arranged diffraction grating formed into the chip's surface. The grating spectrally separates the light to the other waveguides, which act as output waveguides. Separate optical detectors are fabricated on or otherwise associated with the output waveguides for detecting the spectral components of the light. Such a design could conceivably be adapted for parallel DFB lasers formed in such waveguides, but it would not be satisfactory. The DFB gratings would still require precise lithography. Also, the diffraction grating would not be completely decoupled from the optical cavities of the DFB lasers, and the coupled cavities would necessitate a complex and constrained design. Pratt et al. disclose a related bulk-optical design in "Four channel multiple wavelength laser transmitter module for 1550 nm WDM systems, " Electronics Letters, vol. 28, 1992, pp. 1066-1067.
A related design for a multi-wavelength laser array takes advantage of the resonances of the optical cavity associated with the diffraction grating, as has been disclosed by Kirkby et al. in U.K. Patent Application, 2,225,482A: by Kirkby in "Multichannel Wavelength-Switched Transmitters and Receivers--New Component Concepts for Broad-Band Networks and Distributed Switching Systems," Journal of Lightwave Technology, vol. 8, 1990, pp. 202-211; by White et al. in "Demonstration of a Two Wavelength Multichannel Grating Cavity Laser," 12th International Conference on Semiconductor lasers, 1990, pp. 210-211, in "Demonstration of a 1.times.2 multichannel grating cavity laser for wavelength division multiplexing (WDM) applications," Electronics Letters, vol. 26, 1990, pp. 832-834, and in "Crosstalk compensated WDM signal generation using a multichannel grating cavity laser," European Conference on Optical Communications, 1991, pp. 689-692; by White in "A Multichannel Grating Cavity Laser for Wavelength division Multiplexing Applications," Journal of Lightwave Technology, vol. 9, 1991, pp. 893-898; and by Nyairo et al. in "Multichannel grating cavity (MGC) laser transmitter for wavelength division multiplexing applications," Journal of IEE-J Proceedings, vol. 138, 1991, pp. 337- 342. Although Kirkby et al.'s patent application suggests an integrated design, the experimental results of this work involves a separate planar diffraction grating, a bulk lens, and a laser bar of active waveguides formed in a chip. Multiple parallel rib waveguides are formed in the laser bar, each of which could be separately electrically pumped. One waveguide serves as the master amplifier guide while the remainder of the waveguides serves as active reflector guides. None of the waveguides are frequency selective within the overall bandwidth of the laser array, that is, no distributed feedback nor Bragg reflective gratings are formed over the waveguides. Lasing is achieved by simultaneously driving both the master amplifier and a selected one of the reflectors. The combination of master amplifier and selected reflector determine the lasing wavelength because the diffraction grating controls which wavelength propagates between the two. The design ensures that no single rib can lase on its own. The advantage of this approach is that the lasing wavelengths are determined by the relative spatial positions of the waveguides and the diffraction grating, a much easier arrangement to set up than the fabrication of DFB gratings. However, simultaneous multi-wavelength emission creates a problem because the cavities for the different wavelengths, whether in the bulk optical structure or the proposed integrated structure, all include the one active master amplifier. Carrier depletion introduces significant crosstalk between the wavelength channels as they interact in the master amplifier. White et al. in the last paper cited above and Nyairo et al. attempt to suppress the crosstalk with active feedback control or other methods. However, such suppression is considered unsatisfactory because of its active nature. Farries et al. have disclosed an external cavity multi-wavelength laser in "Tuneable multiwavelength semiconductor laser with single fibre output," Electronics Letters, vol. 27, 1991, pp. 1498-1499. Their device uses a bar similar to Kirkby's for a laser array with no frequency determination and relies on lenses and a bulk diffraction grating to define a multi-wavelength cavity.